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Single LED Displays

A simple circuit simulation of an LED, current limiting resistor and power supply

Ken Mardle [kenmar at adr.co.nz] says:

Have purchased several 5,600 mcd white LED's from DSE (cat Z3981, $9.95 ea retail).

They come with a brief data sheet which includes a prominent warning regarding possible eye damage. This warning is not "nannying" - these devices are truly stunning in terms of brightness at 20mA If. They are even uncomfortably bright at 2mA, and usably visible (as a status indicator) at 50 uA.

I predict (as I'm sure others already have) that the days of incandescent bulbs (and probably gas-discharge type lamps) are now numbered. Someday all light sources will be phosphor-coated LED's. Buying shares in Nichia may be a smart move.

They do however have one limitation at present (other than cost). Their emission is broad-spectrum but far from flat - a narrow peak at 460nm (100% relative intensity) with a wider peak at 560nm (50% relative intensity) separated by a minima at 500nm (25% relative intensity). Emission is minimal below 425nm and above 750nm. They can be filtered for other colours but the results will vary depending on the actual colour sought (don't believe people who say you can't filter tham at all - they have soft minds).

Actually they have another limitation - forward voltage drop of around 4.0V. This precludes their use in 3V systems (without trickery) and makes makes current stability in 5V systems doubtful if you just use a resistor for current limiting. You could however make a very nice torch using one of the Linear Tech low-Vin switchmode inverter chips (running in constant current output mode) and a 1.2V or 1.5V battery.

Questions:

See also:


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